A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simplified network architecture. LTE systems also provide seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs).
To establish initial connection with an eNB, a UE first measures downlink (DL) signals to obtain DL synchronization and then sends out a random access channel (RACH) preamble in the uplink (UL) direction. Upon receiving the RACH preamble, the eNB estimates the timing difference and sends back timing advance (TA) information in a random access response (RAR) message. The timing advance compensates for the propagation delay between the eNB and the UE and varies with time, due to UE mobility. During TA maintenance phase, the eNB measures the timing of the received UL data and adjusts the UL timing by TA command. The UE tracks the validity of its UL timing by means of a timing alignment timer (TA timer), which is started or restarted whenever a timing advance is received from the eNB.
Carrier aggregation (CA) is introduced to improve system throughput. With carrier aggregation, the LTE-Advance system can support peak target data rates in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the component carriers.
With Carrier aggregation, a single UE may be assigned radio resources on more than one CC. In some cases, multiple CCs share the same timing advance value and belong to the same timing advance group. In other cases, multiple CCs have different timing advance values and belong to different timing advance group. This is because the DL receptions of different CCs are from different propagation paths. If the time difference between the different paths is larger than a threshold, then the delay becomes non-negligible. As a result, multiple timing advance groups are required such that different timing advance values are applied to different CCs to avoid inter-symbol interference. In one example, the need for different timing advance may arise due to inter-band carrier aggregation, or when transmission for one band is routed via a frequency selective repeater while transmission for another band is not. In another example, DL signals of different bands are routed through different source nodes (e.g., remote radio heads) located some distance apart.
Due to the existence of multiple timing advance groups in a single UE, a comprehensive solution for managing timing advance groups, for maintaining TA timers, and for performing UL synchronization via timing reference signals over different CCs is desired.